Enable PowerPC builds

[nathanchance]

This is blocked on tuxmake upstream:

https://gitlab.com/Linaro/tuxmake/-/issues/108

[nathanchance]

clang-nightly is done, clang-11 requires the above tuxmake issue to be resolved.

[nathanchance]

I am going to close this for now, as it seems unlikely that this will get fixed in tuxmake and it only impacts clang-11 builds. I'll leave the upstream issue open.